Twin roll casting can be set apart from other continuous casting processes in that it is a combined solidification/deformation technique. All of the major competitive processes, such as continuous mold casting, are solidification only, whereafter the cast product is subjected to independent downstream deformation operations. In contrast, twin roll casting involves feeding molten metal into the bite between a pair of counter-rotating cooled rolls wherein solidification is initiated when the metal contacts the rolls. Solidification prior to the roll nip, or point of minimum clearance between the rolls, causes the metal to be deformed, or hot rolled, prior to exiting the rolls as a solidified sheet. The hot rolling operation produces good surface quality, and the rapid solidification due to good thermal contact between the metal and the cooled rolls leads to a very fine grain size, which is preferred for certain applications such as computer hard disks.
There have been numerous patents issued and a large amount of research done on twin roll casting technology. Two early patents showing a twin roll casting apparatus are U.S. Pat. Nos. 3,817,317 to Gilmore and 4,054,173 to Hickam. Although twin roll casting eliminates one or more steps associated with traditional methods, as shown in FIG. 8 of "Continuous Casters for Aluminum Mini-Sheet Mills--An Alcoa Perspective" (1988), twin roll casting has suffered from productivity limitations in comparison. The productivity limits have not been addressed adequately in the prior art, although some solutions have been offered based on experimental work.
In general, the trend has been to produce thinner gauge sheet in the twin roll casting apparatus, which can be rolled at higher speeds due to faster overall strip solidification. Others have conducted studies investigating the effect of strip thickness on the productivity of twin roll casters. Due to problems associated with starting a twin roll caster at thin gauges, it has been determined that the machine must begin casting at relatively thick gauges and the gauge thickness progressively reduced. The gauge thickness is reduced by decreasing the spacing between the rolls, which is typically accomplished by raising the bottom roll. As the rolls are brought closer together, and the strip gauges are reduced, the speed of the rolls can be increased.
Some increase in productivity has apparently been achieved during these experiments. However, the experimental strip widths have typically been limited to 150 mm, or about 6 inches, and reported at speeds only up to 10 m/min, or 15 m/min maximum. In contrast, commercial twin roll casting operations may include strip widths close to 100 inches and may run at much greater line speeds. To date, it is believed that no one has been able to scale up and integrate these promising results in laboratory settings to a larger commercial twin roll casting apparatus in an actual casting line. For example, one of the big problems with casting extremely thin sheet has been the inability to ensure extremely close tolerances of the roll crowns. While a slight deviation from a desired roll crown may be acceptable for casting 6 mm thick strip, the same deviation may be totally unacceptable when casting 1 mm thin strip. And it has proven extremely difficult to ensure a precise roll crown tolerance for actual production-sized rolls.
Therefore, there exists a need for increased productivity in twin roll casting machines and, specifically, a need to solve the problems associated with converting experimental results into a practical commercial unit.